The present invention is directed to radar systems. It has particular application to over-the-horizon target detection and tracking from mobile platforms.
Although radar systems have been extensively studied and improved for over half a century, the goal of acquiring over-the-horizon targets from mobile platforms such as ships has remained elusive. The difficulty arises from two major problems.
The first problem is surface-wave attenuation. In order to obtain the desired directional resolution, the wavelengths used for radar by mobile platforms such as ships have been in the microwave range. But surface-wave mode propagation of microwaves is nonexistent for all practical purposes, so microwaves do not provide over-the-horizon target acquisition. On the other hand, in order to obtain enough directionality with the longer-wave radiation for which surface-wave propagation is significant, it has been necessary to employ phased-array systems that, because of the irregular topology of the typical ocean vessel, have heretofore been impractical for such platforms.
The second problem is noise. A radar operating in a surface-wave mode at sea encounters a lot of reflections from ocean waves, which in the aggregate greatly overwhelm the returns from all but the largest targets of interest. The presence of many over-the-horizon radiation sources in the lower-frequency regions of the spectrum compounds this problem.
As a consequence of these difficulties, low-flying targets located over the horizon have been essentially immune to detection from mobile platforms.